The speed at which snow falls appears slow and graceful, but the exact rate of descent is not a simple, fixed number. A snowflake’s journey through the atmosphere is a complex physics problem constantly influenced by its surroundings. The speed of a single snowflake can vary drastically, ranging from a near-halt to a brisk pace. This variability depends on a variety of intrinsic and extrinsic factors encountered on its way to the ground.
Defining the Speed: Terminal Velocity
The fundamental concept that governs the speed of any falling object in the atmosphere is called terminal velocity. This maximum speed is achieved when the downward force of gravity acting on the object is perfectly balanced by the upward force of air resistance, or drag. Once this balance is met, the object stops accelerating and falls at a constant rate.
For a typical snowflake, this balance occurs at a surprisingly slow speed compared to raindrops. The measured terminal velocity for most snowflakes generally falls within the range of about 1 to 4 miles per hour (0.5 to 2 meters per second).
This slow descent rate is a direct consequence of a snowflake’s low mass and large surface area relative to its weight. The porous, non-aerodynamic shape of snow crystals creates a high amount of drag force. Terminal velocity represents the theoretical maximum speed a given snowflake could achieve if falling perfectly vertically in completely still air.
Internal Factors: Snowflake Shape and Mass
While terminal velocity provides the baseline speed, the specific properties of each snowflake determine where it falls within the speed range. The most significant internal factors are the particle’s shape and its mass. These two characteristics dictate the ratio of drag to weight, fundamentally altering the descent rate.
Dendrites, the classic, highly branched, stellar crystals, are known for their large surface area and low density, which maximizes air resistance. These light, fluffy structures have a relatively slow terminal velocity because the drag force quickly compensates for their minimal mass. In contrast, denser forms of frozen precipitation, such as graupel or ice pellets, are more compact and spherical.
Graupel forms when supercooled water droplets freeze onto a snow crystal, creating a heavier, more massive particle with a smaller surface area-to-mass ratio. This lower drag relative to weight means that graupel falls much faster than delicate stellar dendrites.
Aggregation, the clumping of multiple crystals into a single large snowflake, further complicates the speed calculation. While aggregation significantly increases the total mass, it also creates a massive, irregular shape that dramatically increases the surface area and air resistance, often resulting in a moderate speed increase compared to single flakes.
External Factors: Wind and Atmospheric Density
Beyond the intrinsic properties of the snowflake, external atmospheric conditions constantly modify the actual speed and trajectory of the falling snow. Wind is the most obvious factor, introducing significant horizontal movement, but it also affects the vertical fall speed.
Turbulence and vertical air movement can cause the actual settling speed of snowflakes to vary substantially from their still-air terminal velocity. Powerful updrafts, which are rising currents of air, can temporarily halt or even lift light snowflakes, reducing their effective rate of descent to zero. Conversely, downdrafts can accelerate the particles, causing them to hit the ground faster than their calculated terminal velocity.
Air density, related to temperature and altitude, also plays a role in determining the drag force. Colder air is typically denser than warmer air, meaning it offers slightly more resistance to the falling snowflake. This increased resistance slightly reduces the terminal velocity, but the effects of wind and vertical air currents are generally much more pronounced.